The ion conductivity of numerous solid electrolyte materials has resulted in the application of solid electrolyte electrochemical cells for numerous gas measuring applications. Solid electrolyte compositions have been incorporated in gas sensing apparatus to measure oxygen, combustibles, pollutants, i.e. SO.sub.2, CO.sub.2, etc. and have provided a basis for product ranging from medical instrumentation to industrial process and stack gas analyzers. A solid electrolyte electrochemical cell develops an electrical signal, current or voltage, on the basis of ion conductivity which is a function of the content of a particular gas species of interest within a monitored gas environment. The application of a solid electrolyte electrochemical cell exhibiting oxygen ion conductivity to monitor the oxygen content of a gas environment is described in detail in U.S. Pat. No. Re. 28,792 which is assigned to the assignee of the present invention and incorporated herein by reference. The solid electrolyte electrochemical cell, which consists of an oxygen ion conductive solid electrolyte having a sensing electrode disposed on one surface and a reference electrode disposed on an opposite surface, develops an EMF signal in accordance with the Nernst equation in response to a difference in the oxygen partial pressure between the sensing and reference electrodes. In order to eliminate the oxygen partial pressure at the reference electrode as a variable in the equation, a stable or known oxygen reference environment is maintained in contact with the reference electrode. This reference environment is isolated, through appropriate sealing means, from the measured gas environment contacting the sensing electrode. The same basic mode of operation of a solid electrolyte electrochemical cell as employed for the measurement of combustibles is described in U.S. Pat. No. 4,158,166, which is assigned to the assignee of the present invention and incorporated herein by reference. In this embodiment, the electrochemical cell is operated in a pumping mode to introduce oxygen from the reference source to combustibly react with combustible constituents at the sensing electrode with the cell current developed as a result of the ion conductivity providing an indication of the combustibles content of a measured gas environment. The application of solid electrolyte electrochemical cells employing a stable reference gas environment for developing an EMF signal indicative of pollutants such as SO.sub.2, CO.sub.2, NO.sub.2, etc. is described in detail in issued Canadian Pat. Nos. 1,002,599 and 1,040,264 which are assigned to the assignee of the present invention and incorporated herein by reference.
The sealing, or isolation, of the measured gas environment from the reference gas environment poses significant practical problems in developing a trouble-free gas-sensing apparatus due to the fragile nature of the ceramic material comprising the solid electrolyte element. This problem is further compounded when developing a high temperature solid electrolyte gas-sensing apparatus, particularly when it is intended for monitoring the gas constituents of an industrial corrosive environment.
Typically, the commercial products presently available addressed this problem in one of two ways. In a first approach, the electrolyte material is limited to a relatively small disc-shaped solid electrolyte element having electrodes disposed on opposite surfaces thereof with the electrochemical cell then being sealed within a relatively strong mechanical tubular housing by an appropriate bonding technique, i.e., brazing. This minimizes the chances for mechanical fracture of the ceramic material but poses potential gas leaks through the seal. A second approach is to utilize a closed end solid electrolyte tube with the electrodes being disposed on opposite surfaces of the closed end. The tube extends into a relatively cool and friendly environment before a transition is made through a flange or an adapter. This approach essentially eliminates the seal problem but exposes the fragile ceramic material to mechanical damage.
Another drawback encountered in presently available commercial products is the combination of rigid and flexible tubing employed to introduce the reference gas to the electrochemical cell and to exhaust the spent reference gas from the sensor apparatus. The flexible tubing is subject to gas leaks, particularly where mated to the rigid tubing, and to thermally induced damage.
It is an object of this invention to provide a gas sensing apparatus which vents the reference gas from the sensing apparatus into the process gas region thus eliminating the flexible tubing and hermetic seals, heretofore common in commercially available products.